Large capacity air conditioning systems have used centrifugal compressors with guide vanes at the compressor inlet, which vanes are adjustable to regulate both the direction (or "swirl") of the incoming gas and also produce a pressure drop which is a function of the vane position. These vanes, sometimes termed pre-rotation vanes (PRV), are thus adjusted to vary the capacity of the compressor. In the condition of wide-open vanes (WOV) a small change in the vane position does not have a substantial effect on the compressor head or capacity. When the PRV are nearly closed a slight change has a very substantial effect on the compressor capacity, and could even send the compressor into surge if care is not taken in adjusting the vane position. Because capacity control solely by changing PRV position is an inefficient method of capacity control, there have been attempts to regulate system capacity only by governing the speed of the electrical motor driving the compressor. If speed control is utilized as the only means of regulating the capacity, the compressor can only be operated down to about 70% of its full load. This is economically impractical for large installations, since for substantial periods of time the load is below 70% of the full load value.
Accordingly various attempts have been made to combine adjustment of the PRV with regulation of the motor speed, which can successfuly reduce the load down to about 10% of full load. One significant early effort in this direction is taught in U.S. Pat. No. 3,355,906, entitled "Refrigeration System Inlcuding Control for Varying Compressor Speed", which issued Dec. 5, 1967, and is assigned to the assignee of this invention. In that arrangement the motor speed was adjusted as a function of the ratio between the compressor suction pressure and discharge pressure, and the PRV were adjusted in relation to a signal derived from the temperature of the heat exchange medium at the discharge of the evaporator. Most similar efforts since that time have used the pressure ratio of the compressor to regulate the motor speed, and derivation of this pressure information has proved difficult and expensive in practice. In addition it has been found that the regulation of the pre-rotation vanes is a complex function, which has not been effectively controlled to avoid surge and provide most efficient operation by sensing of only a single variable condition.
It is therefore a primary object of the present invention to provide a control system for regulating a refrigeration system, such as one including a centrifugal compressor, in which not only are the motor speed controlled and PRV position adjusted, but this speed control and vane adjustment are achieved over an optimum path of control with minimum energy expended.
A related important aspect of the invention is the provision of such a control system which achieves such energy effective system operation, and at the same time avoids surge.
Of prime importance is the provision of such a control system, which is not only capable of installation with new equipment, but is readily installed on existing systems to achieve the optimum control path while avoiding surge in those systems.
Still another important object of the invention is linearization of the PRV action, through a controller with a non-linear duty cycle to produce more uniform system control.
Another important object of the invention is to achieve such precise system operation over a wide load range without the necessity of sensing compressor pressures to derive the compressor head value.